The electric power supply for consumers in a motor vehicle makes special demands on the voltage regulator used because the on-board voltage fluctuates in a relatively wide range and drops significantly when heavy loads are switched on, for example. Especially when starting the engine, as long as the starter is in operation, these voltage drops may be considerable. In supplying electric power to voltage-sensitive consumers such as a controller or a microprocessor and especially in supplying electric power to an automotive PC to be used in future motor vehicles, it is necessary to ensure that no voltage drop will be so great as to lead to a reset of the controller or the PC.
One possibility of supplying a stabilized power supply voltage for controllers or other voltage-sensitive consumers is to use complex switched-mode power supplies. Such switched-mode power supplies are necessary to minimize the resulting power loss, but when starting the engine, it can nevertheless occur that there is a great dip in the power supply voltage; in this case, the switched-mode power supply parts are shut down and cause an unwanted reset of the PC.
A circuit configuration for generating a power supply voltage which protects sensitive electronic consumers such as microprocessors but will also shut down when the voltage is too low is described in German Patent No. 33 35 200. With this power supply voltage circuit, two downstream voltage converters are used, making available an unstabilized consumer voltage as well as a stabilized consumer voltage. The unstabilized consumer voltage is generated in a separate voltage converter and is used as the input voltage for the downstream second voltage converter which includes a voltage regulator. Both voltage converters are active at the same time. In addition, there is a low voltage detection which shuts down the consumer when the power supply voltage drops below an allowed minimum level.